1. Field of the Invention:
This invention relates to light measuring circuits, and, more particularly, to light measuring circuits for detecting light reflected from an object being photographed with illumination of flash light projected from a strobe device.
2. Description of the Related Art:
As the conventional method for measuring light from the strobe device, it has been customary that as shown in FIG. 1, a condenser is connected in series with the light receiving element, such as a silicon photo-cell, so that when a switch connected across the condenser is opened at the same time as the start of emission of flash light from the strobe device, the photocurrent of the light receiving element starts to accumulate on the condenser, whereby the voltage stored is taken as the light value.
The use of this method to derive a proper flash exposure by controlling the amount of flash light emitted in accordance with the output of the photo-sensitive element requires that the film sensitivity and aperture size be factored into the flash exposure value. For this purpose, it has been the prior art practice that the capacitance of the condenser is made variable or the critical level for the voltage stored on the condenser is is made variable, depending on the film sensitivity and aperture values.
However, the prior art method has drawbacks that the changeover means is necessarily included to increase the complexity of the structure of the mechanisms, and the precision accuracy of the responsiveness changes from means to means, deteriorating the accuracy of light measurement, that while the dynamic range of light is wide, a large limitation is placed on the range of variation of the voltage, so that S/N cannot be taken at a value large enough to discriminate the voltage, and further that in the case when the ambient light enters the light receiving element, the measurement for the flash light cannot accurately be performed. The first two problems can be eliminated by a method that, as shown in FIG. 2, the photocurrent is first logarithmically compressed, and when the photo-current is then expanded by a transistor, the emitter of the transistor is set to a voltage corresponding to the film sensitivity and aperture value so that the current expanded by the transistor is determined by the film sensitivity and aperture value. It is by this current that the condenser is charged. The voltage stored on the condenser is then tested.
Even by such a method, however, the third problem could not be overcome. In more detail, if the flash light (an alternating current light) and the ambient light (a direct current light) impinge at the same time on the photosensitive element, photocurrents I.sub.AC and I.sub.DC are produced, respectively, and the amplifier of FIG. 2 produces an output voltage Vout expressed by: ##EQU1## where I.sub.S is the backward saturation current of the diode.
Now, for the purpose of simplifying the explanation, assuming that the voltage at the emitter of a transistor, whose base is connected to the output of the amplifier, is zero volt, the collector current of the transistor becomes EQU I.sub.C =I.sub.DC +I.sub.AC
Because the condenser is charged by this current, the discrimination of the voltage of the condenser cannot be reflected to the measurement of the flash light only.
To eliminate this problem, a filter may be used to cut off the signal of the direct current light, leaving the signal of the alternating current light to the readout of the measured light, as disclosed in U.S. Pat. No. 4,291,979 and Japanese Patent Publications Nos. 47-2756, 47-4088 and 47-4089. In these patents, to be sure, the alternating current component can be obtained out of the photo signal at the output of the light measuring circuit. Yet, upon consideration of the so-called fill-in flash photography, it is desired to add a fraction of the direct current component (of the ambient light) to the alternating current component (of the flash light) when the illumination of the scene is measured. The prior art cannot cope fully with this end. For example, in the above-described U.S. Pat. No. 4,291,979, the output of the light measuring circuit is processed by a very complicated circuit when the direct current component is taken into account before a proper flash exposure value is obtained.